Direction finder

ABSTRACT

AND AMPLITUDES AND GENERATE THEREFROM AN AMBIGUITY-FREE MEASURE OF THE AZIMUTHAL ANGLE OF THE ELECTROMAGNETIC PULSE SOURCE.   A DIRECTION FINDER FOR DETERMINING THE AZIMUTHAL LOCATION OF A SOURCE OF ELECTROMAGNETIC WAVE ENERGY SUCH AS THE ELECTROMAGNETIC PULSE RESULT FROM A NUCLEAR DETONATION OR LIGHTNING STROKE. SUCH DETERMINATION IS ACCOMPLISED USING A PAIR OF ORTHOGONALLY RELATED DIRECTIONAL ANTENNAS TOGETHER WITH DIGITALLY IMPLEMENTED SIGNAL PROCESSING MEANS WHICH DERIVE FROM THE RECEIVED SIGNALS MEASURES OF THEIR RELATIVE PHASES

ilnited States Patent [1 1 (loleman, et a1.

[54] DIRECTION FINDER Primary Examiner-T. H. Tubbesing [75] Inventors:Fred J. Coleman, Jr., North Atmmey- Carl Baker et Syracuse; Richard C.Weischedel, Camillus, both 0f N.Y. [57] ABSTRACT [73] Assignee: GeneralElectri C p y, A direction finder for determining the azimuthal loca-Syracuse, NY. tion of a source of electromagnetic wave energy such [22]Filed; June 30, 1971 as the electromagnetic pulse resulting from anuclear detonation or lightning stroke. Such determination is 1 Appl'accomplished using a pair of orthogonally related [52] US. Cl 343/119directional antennas together with digitally imple- 51 Int. Cl G015 3/30mente ign processing means i h rive from [58] Field of Search 343/119the received signals measures of their relative phases and amplitudesand generate therefrom an ambiguityv [56] References Cited free measureof the azimuthal angle of the electro- UNITED STATES PATENTS magneticPulSe 2v992,428 7/1961 White 343/119 10 Claims, 5 Drawing Figures3,34443O 9/1967 Hildebrand 343/119 X 3.496.565 2/1970 Jenkins 343/119POLARITY xi men DETECTOR OCTANT PRESET OCTANT OUTPUT [SELECTOR LIP-DOWNCOUNT nmmm 47 37 33 B D 1 AMPLIFIER ueowu 2192 x COUNTER X NORMALIZER XI51 1 x LOOP 29 Y POLARITY MAGNITUDE E lYl ATIO /w LOOP t E gr lCON/VERTOR l ml i AMPLIFIER HOLD 44 53 59 NORMALIZER (ABSOLUTE) 25 VALUECLOCK 4. e LL PUFS E 57 TNIOTLQQ/ALIZER ENAB; N02 233- 55) AB Q3 E N LEDETECTOR ENABLE I DELAY TIME PLEUEIX 1 Hg/f PULSE I PULSE DETECTOR CLEARSTART THRESHOLD i i l5 OMNI PATENTEB nuvzo 197s sum 3 0f 4 THEIRATTORNEY.

DIRECTION FINDER This invention relates generally to direction findersand more specifically to such finders capable of determining theazimuthal location of an electromagnetic wave energy source bymeasurement of phase and amplitude characteristics of signals receivedfrom the source. While useful also for locating other electromagneticenergy sources such as lightning strokes, the direction finder of theinvention affords particular advantage as applied to location of nuclearevents by sensing the electromagnetic pulses which accompany them andderiving azimuthal measurements from the pulse characteristics sensed.

The waveform, frequency and other characteristics of the electromagneticpulse which accompanies a nuclear detonation has been treated at lengthin the literature, as for example in the paper by Johler andMorgenstern, entitled Propagation of the Ground Wave ElectromagneticSignal, With Particular Reference to a Pulse of Nuclear Origin,published in the December 1965 IEEE Proceedings, Vol. 53, No. 12, Pgs.204 32053. As will be apparent from the description of the ground waveelectromagnetic signal given in this and other references, its use forsource location is difficult of accomplishment using conventional radiodirection finding equipment. Principal among the difficulties involvedare the very short duration of the pulse, which is much too brief topermit any operator manipulation of receiving antenna orientation duringthe time of reception of the pulse, as is required by many conventionalradio direction finders, and the extreme dynamic range of the pulsewhich makes it very difficult to obtain meaningful measures of pulseamplitudes and waveform characteristics as needed for azimuthal angledeterminations.

While the currently most commonly used radio direction finders requireoperator manipulation of loop antennas or other directional wavesensors, the prior art includes a number of direction finder systemscapable of deriving azimuthal angle measurements without suchmanipulation; the systems described in US. Pat. Nos. 3,344,430 toHildebrand and 3,490,024 to Sherrill et al are representative of knownsystems of this type. Another such system employs crossed loop antennaswith each loop controlling the deflection drive of a cathode ray tubedisplay in one of its two orthogonally related coordinates, and in somesystems of this kind there is provision for resolving the 180 ambiguitywhich otherwise exists by intensity modulating the CRT in accordancewith the signal received by an associated omnidirectional antenna.

When used for source location in nuclear event detection systems,crossed loop direction finders of this general kind have encounteredserious difficulties in the problem areas previously mentioned,particularly in resolving the 180 ambiguity in azimuth even with aid ofan omnidirectional sensor input, and in accomodating the extreme dynamicrange of the signals received. The several known approaches to thislatter problem have not proven entirely satisfactory. As generallypracticed, for example, logging techniques require close matching ofdiodes and precise temperature control, and because conventional signalnormalization techniques require time before the normalization canbecome effective they require delay elements and more complex circuitryto introduce this time delay and factor it into control of signalprocessing operations.

The present invention relates to direction finders of the general classjust described, and has as its primary objective the provision of suchdirection finders affording significant performance advantagesparticularly as applied in nuclear detection equipments for determiningthe azimuthal location of a nuclear event. As will become apparent asthe description of the invention proceeds, it affords such desiredcharacteristics as the capability to determine azimuth location of apulse source radiating only an extremely short duration pulse, theability to accommodate an extremely wide dynamic range of pulseamplitudes, and the ability to provide a substantially instantaneousreadout of azimuth angle free of an ambiguities and wih a degree ofaccuracy which may achieve relatively high levels without unduecomplication of the system circuitry. In its preferred embodiment thesystem circuitry is implemented in digital form facilitating theachievement of relatively high resolution of azimuth angle by additionalparalleling of certain components which are relatively simple incircuitry and relatively low in cost.

It is accordingly an object of this invention to provide a directionfinder capable of providing substantially instantaneous indication ofthe azimuthal location of an electromagnetic wave energy source, with nomanipulation or other operator input required to obtain anambiguity-free indication of source azimuthal angle. it is also anobject of the invention to provide such direction finder capable ofderiving the azimuthal angle determination with an electromagneticenergy pulse of extremely short duration and of extremely wide dynamicrange, thus adapting the direction finder to use in nuclear eventlocating applications. Still another object of the invention is theprovision of a direction finder of this kind which is digitallyimplemented so as to enable direct output of the measured azimuthalangle in digital form, and to enable achievement of desired degree ofaccuracy of the azmithal angle measurement without undue complexity orcost of required circuitry for its implementation.

SUMMARY OF THE INVENTION In its preferred embodiment as hereinafterdescribed, the direction finder of this invention employs a pair ofdirectional receptors which conveniently may take the form of loopantennas arranged in orthogonally related orientation such that theirrespective directions of senstivity are along mutually perpendicularaxes which for convenience may be termed the X and Y axes. Preferably athird receptor of omnidirectional sensitivity characteristic, such asprovided by simple dipole or whip antenna, is included to provide anondirectional amplitude signal which is used for signal normalizationand thresholding, and where necessary also for resolving any ambiguityin direction indication. Each of the X and Y channel signals is coupledthrough a normalizer which attenuates both signals identically in two ormore discrete steps in response to a normalization control input whichis derived from the nondirectional amplitude signal, to thus maintainthe X and Y channel signals at manageable and identically matchedattenuation levels through their respective processors.

After normalization of the X and Y channel signals, the polarity of eachof these signals is sensed and their polarities transmitted to selectormeans, which derives therefrom the quadrant in which the source islocated. The X and Y channel signals are applied to absolute valuecircuits so that both signals have the same polarities. An amplitudecomparison of the [X] and [Y] signals then is made and its resulttransmitted to the selector means, which determines from this inputwhich of the two octants in the previously selected quadrant the sourcelies in. Finally, the [X] and [Y] channel signals are ratioed to derivea measure of the angular position of the source within the octant thusselected, this preferably being accomplished by digital counter meanswhich are set by the octant selector to a count corresponding to theidentified octant and which then counts up or down from this presetcount in accordance with the ratio of the two signals.

When the counter has completed this operation it will have stored anumber representative of the azimuthal angle to the point of origin ofthe electromagnetic wave energy sensed. This information can be shiftedinto permanent storage or shown on suitable display means as preferred.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherunderstood and its various objects, features and advantages more fullyappreciated by reference to the appended claims and to the followingdetailed description when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a plot of EMP wave amplitude against time, for a nuclear eventof representative magnitude and distance from the point of measurement.

FIG. 2 is a compass rose illustrating the basis on which octantselection is made by the direction finder of FIG. 3.

FIG. 3 is a block diagram ofa direction finder system in accordance withthe invention.

FIG. 4 is a schematic circuit diagram of certain subsystems of thedirection finder system of FIG. 3.

FIG. 5 is a schematic circuit diagram of certain other subsystems of thedirection finder of FIG. 3.

FIG. 6 is a truth table illustrating the operation of the octantselector in presetting the up-down counters in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1, which is based on theEMP ground wave data contained in the .Iohler-Morgenstern paperreferenced above, illustrates a typical electromagnetic pulse waveformat approximately 50 miles from the point of detonation of the nucleardevice from which radiation derives. As shown, the waveform goesinitially sharply negative, then reverses polarity, becomes positive,and slopes relatively slowly toward a second crossing of the zero axis.The entire waveform through its two zero crossings requires only a timeperiod of less than 100 microseconds as reported in theJohler-Morgenstern paper, at this approximate 50 mile distance from itspoint of origin.

While the time duration of the wave is dependent on the distance fromthe source to the point at which it is to be observed, at distanceswithin the range of interest here the time duration of the pulse wouldalways by too brief to enable direction finding using conventionalsingle loop equipments requiring operator manipulation. In accordancewith the invention, direction finding is accomplished without operatormanipulation or other adjustment of sensor elements, and is completedwithin the relatively very brief period of a typical nuclear EMPwaveform.

To accomplish this the system of this invention comprises a pair ofidentical loop antennas as shown at 11 and 13 in FIG. 3, the two loopshaving orthogonally related polarizations or sensitivity patterns whichare respectively oriented in X and Y directions, where the Y directionis defined as North-South and the X direction as East-West in theparticular embodiment being described. It can readily be shown that theY loop will give an output which is proportional to the cosine of theazimuth angle to a source of electromagnetic radiation incident upon thetwo loops, in accordance with the relation:

Ey E cos 0 Since the X loop is oriented at right angles to the 0reference, it gives an output which is proportional to the sine of theazimuth angle, that is,

Ex E Sin 0 The effect thus is to resolve a received signal into its Xand Y components, yielding sufficient information to determine theazimuth of the received signal. However, use of the signal amplitudesalone is not sufficient to determine the azimuthal angle withoutambiguity; the polarities or relative phases of the signal voltages mustalso be factored into the angle determination to resolve ambiguitiesotherwise present.

This can perhaps most easily be understood by reference to the compassrose of FIG. 2. As will be obvious from inspection of FIG. 2, thereexists within each octant of the compass rose a unique combination of Xand Y signal polarities and of the relative magnitudes of absolutevalues of the X and Y signals. Thus in the first octant, from 0 to 45,the X and Y signals both are positive and the absolute magnitude of theY signal exceeds that of the X; in the second octant, from 45 to the Xand Y signal polarities again are both positive but the absolute valueof the X signal now exceeds that of the Y. Further, since the ratio ofthese absolute magnitude signals varies across each octant, measurementof this signal ratio enables a fine measurement of the angular positionof the source within the octant. In the first octant, for example, theratio of the Y signal absolute value to that of the X varies from zeroat 0 azimuth to unity at 45; the source location accordingly is fixedwithin this octant as the angle whose tangent is equal to the value ofthis ratio.

From FIG. 2 it is apparent that azimuth angle could be determined firstby identifying the quadrant in which the source lies by sensing thepolarities of the two loop signals, then fixing the azimuth angle withinthat quadrant by ratioing the absolute magnitudes of the two loopsignals. Such technique is difficult to implement, however, becausewhichever of the two signals is divided into the other to obtain theirratio will, at one extreme of the quadrant, approach zero which ofcourse will cause the ratio to approach infinity. The processing of suchundefined ratios, particularly in digital implementations, is complexand difficult.

To avoid this, the direction finder of this invention uses a combinationof polarity sensing and magnitude comparison to identify the octant inwhich the source lies, then fixes the angle within that octant byratioing the loop output signals. Since the angle over which ratioingneed be accomplished is only the 45 angle of the preselected octant, itis possible by proper control of the signal ratioing means to assurethat the smaller of the two signals always is in the numerator position,thus assuring that their ratio always will fall between zero and unity.Ratios within this range may readily be processed as necessary todetermine azimuth angle within the preselected octant.

The receptor or antenna loops preferably are of dimensions small ascompared to the EMP signal wavelengths, which span a relatively wideband corresponding to the frequency band from about Hz to perhaps 150KHz, peaking at approximately 12 KHz. The magnitude of the antenna loopcurrent then will be proportional to the sensed signal amplitude and tothe effective area enclosed by the loop, i.e., its enclosed area asprojected onto the plane perpendicular to the direction of thepropagating signal.

If, as in the nuclear event detection application, it is known what thepolarity of the initial half cycle of the observed waveform will be,then the polarities of the X and Y loop signals may be determined withno ambiguity without need for any additional sense input. However, wherethe observed waveform may initially be of either polarity, an ambiguitywill exist and to resolve such ambiguity a third receptor,omnidirectional in sensitivity pattern, may be provided as indicated at15. Even in the absence of need for a nondirectional input for thepurpose of resolving such ambiguity, the omni desirably is included asit serves additional purposes as will be explained.

Among these additional purposes is the control of timing of the polaritydetection, magnitude comparison and ratioing operations. Such control iseffected through a start threshold element 17 and delay elements 19, 21and 23, which respectively time certain of the operations of the system.Start threshold 17 preferably is set so as to output its START signalwhenever the input signal sensed by the omni antenna substantiallyexceeds the noise; typically this threshold will be reached within a fewmicroseconds after the start of the EMP pulse waveform. The STARTsignal, delayed at 19 by a few microseconds as indicated, becomes TimePulse No. l and triggers an enable element 25 which actuates polaritydetection means 27 and 29 included in the X and Y signal processingchannels respectively. The X and Y polarity signals thus derived aretransmitted to an octant selector 31 and also to X and Y absolute valueamplifiers 33 and 35, respectively.

The X and Y signal inputs to these absolute value amplifiers aretransmitted through normalizers in he X and Y channels respectivelydesignated 37 and 39. These normalizers operate under control of anormalization signal derived by the normalizer-detector 41 from thenondirectional amplitude signal sensed by the omni antenna l5, and serveas a pair of matched variable attenuators which are simultaneouslyadjusted in response to the normalization signal to attenuate both X andY signals identically. More specifically, and as more fully detailedhereinafter, normalizers 37 and 39 may comprises an identical pair ofresistance networks with electronic switching means providing commoncontrol of both in response to the normalization signal, so as to outputnormalized X and Y signals each representing an identical fraction ofthe input, of value selected to hold the larger of the two signalswithin predetermined dynamic range limits.

The signal output from each of the normalizers 37 and 39 is amplified inthe corresponding absolute value amplifier 33 or 35 and applied to amagnitude comparator 43 which determines whether the [X] or [Y] signalhas the greater amplitude, and outputs a signal accordingly to a holdcircuit 44. This circuit when energized by Time Pulse No. 2, which isapplied to it through an enable circuit 45 in response to the STARTsignal as delayed at 21 by about 10 microseconds as indicated, sensesand holds the magnitude comparator output through the duration of theoperating cycle. The signal thus held is applied as a logic X Y or X Ysignal to the octant selector 31 and, with its other inputs, causes theselector to output PRESET OCTANT and UP-DOWN COUNT control signals to aBCD up-down counter 47.

The octant identification and count up or count down decisions by octantselector 31 are made by examining the outputs of the two polaritydetectors and of the magnitude comparator. If the input signal arrivedat an azimuth an gle between 0 and 45, for example, both X and Y signalswill have a normal positive polarity and the absolute value of Y isgreater than the absolute value of X. For this particular input, the BCDupdown counter will be preset to zero and the counter programmed tocount up therefrom. As another example, if the input is between 45 andthe X and Y signals still will have positive polarity but here theabsolute value of X will be greater than that of Y. The BCD up-downcounter then will be reset to a count of 90, and the counter programmedto count down therefrom, i.e., downward from 90 towards 45. Theseoperations of the BCD up-down counter will be covered in greater detaillater with reference to the truth table of FIG. 6, which illustrates itsprogramming.

The switch drive 49 and reversing switch 51 operate in response to the XY or X Y signal from magnitude comparator 43 to connect whichever it hasfound to be the larger of the [X] and [Y] signals to one of the twoinputs to a digital comparator 53, and to connect the smaller of the [X]and [Y] signals always to the other of the two comparator inputs. TheseX and Y channel signals also are applied to a positive level detector 55which is triggered, when the mean value of the higher and lower inputsto the digital comparator exceeds a preset or threshold value, toenergize an enable circuit 57. This in turn energizes a clock 59initiating readout of the results of the digital comparison at 53 intothe BCD up-down counter, and it applies also a mode control signal tothe counter preparatory to such readout. Should the [X] and [Y] signalsas applied to positive level detector 55 not reach this threshold value,the detector still will be triggered to initiate readout of the digitalcomparator into the counter by Time Pulse No. 3 which is inputted todetector 55 after delay at 21 and 23 for a time greater than onehalfcycle of the waveform signal.

Upon completion of counter operation, the count then standing may beoutputted to any convenient recording or display means, and representsthe azimuthal angle of the source of the signal which triggered theSTART threshold and the ensuring direction finding process. It will benoted that all measurements necessary to the source directiondetermination are completed within one cycle of the waveform signal,thus enabling completion of the direction finding operation even withextremely short pulses such as illustrated in FIG. 1. As will also benoted from that figure, the system operates to derive the octant data,i.e., to detect signal polarities and to determine which of the X and Ysignals has the greater amplitude, during the first or negativehalf-cycle of the waveform, and measures the ratio of the amplitudes ofthe two signals during the second half-cycle. This enables normalizationof the signals to be completed before making any signal amplitudemeasurements, and allows sufficient time for transients to subside priorto signal ratioing for degrees measurement. Further, this timingsequence avoids the taking of measurements near the zero crossover pointat which signal amplitudes are difficult to measure accurately and atwhich signal ratios may become indeterminate, and places the differentsignal sensing steps at points along the waveform at which they can mosteffectively be accomplished.

With reference now to FIG. 4, the X channel signal processing circuitry,including the signal normalizer 37, the X signal polarity detector 27and absolute value amplifier 33, are shown together with certain of thecommon or omni channel components including the normalizer detector 411,magnitude comparator 43, the reversing switch drive 49 and reversingswitch 51. The Y channel signal processing circuitry is identical tothat of the X channel, so in the interests of simplicity this Y channelcircuitry is shown simply as a block 60 which will be understood toinclude the same components as now to be described with reference to theX channel processor.

The X channel antenna loop has its opposite ends connected totermination networks 61 and 63 which as shown comprise resistance andcapacitance elements of values such as to properly load the antenna andprovide the desired resonant frequency and bandwidth, and which formparts of the polarity detection and normalization circuits 27 and 37,respectively. The connections of these circuits to the antenna loop aresuch that for the first half cycle of quadrant l signals, the positiveend of the loop is that connected to the normalization circuit 37 andthe negative end of the loop, the signal on which will for conveniencebe termed the inverted" signal, is connected to the polarity detectioncircuit 27.

In the polarity detector 27, the inverted X signal is amplified by anon-inverting amplifier 67 which conveniently may take the form of adual sided operational amplifier enclosed in a negative feedback loopthrough a voltage divider which feeds back to its negative input aportion of its output, to thereby control amplifier gain. The amplifiedoutput is capacitor-coupled to the negative input of a second suchoperational amplifier 69 the positive input to which is connected toground. This amplifier 69 accordingly will have an output which remainslow when the inverted X signal is positive and which will be high whenthe inverted X signal is negative. The amplifier output is connected tothe set" input of a flip-flop 71 having as the input to its clock"terminal Time Pulse No. l, which it will be recalled occurs a fewmicroseconds after the START signal.

if the input from amplifier 69 is high when Time Pulse No. l isreceived, the l terminal of the flip-flop will go high and the terminallow. This denotes that the inverted X signal is negative and results inthe out put of a logic X signal on lead 72; were the X signal positivethe 0" terminal would be high resulting in the output of a logic Xsignal on lead 73. The signals on these leads thus constitute a polarityindication, and are transmitted to the octant selector of FIG. forutilization there in the manner to be described.

The X signal also is applied to the absolute value amplifier circuit 33,in which it is diode coupled to the base of a transistor 74. If theinverted X signal is negative for this first half cycle, the resulting Xinput to the transistor base electrode will switch the transistor offand its collector and the gate of an FET analog switch connected theretowill go negative, substantially to the negative supply voltage (-V).Such negative gate voltage will turn the switch 75 off, thus isolatinglead 77 from the ground connection otherwise provided through this switch. If the inverted X signal is negative, there will be no X input totransistor 73 and the transistor and PET analog switch 75 will remainon, effectively shunting lead 77 to ground through the switch.

The grounding of lead 77 controls the polarity of output of a dual sidedoperational amplifier 79, which is arranged to provide a positive outputfor the first halfcycle of the X signal, that is the half cycle beforezero crossing, and a negative signal for the second half cycle. Toaccomplish this, the amplifier input, through capacitor 81, is coupledto one of the two amplifier inputs through two series connectedresistors 83 and 85, both of approximately the same resistance valuewhich is selected to be relatively high as compared to the internalresistance of the FET analog switch 75. Then when the transistorconducts and lead 77 is grounded therethrough, the input terminal of theamplifier 79 to which these resistors connect effectively is grounded.Under these conditions the input through resistor 87 to the other inputterminal of amplifier 79, and its voltage feedback through resistor 89,will cause the amplifier to function as an inverting amplifier providinga positive signal output for a negative signal input. When lead 77 isisolated from ground by switch 75, both input terminals of amplifier 79then are effectively tied to the same potential, the amplifier inputimpedance being much greater than that of resistors 83 and 87, and theamplifier accordingly will function as a voltage follower providing anoutput of the same polarity as its input.

The input signal to operational amplifier 79 is the X channel signaltaken from the positive end of the X antenna loop and coupled through aphase control network 65 comprising parallel connected resistance andcapacitance elements providing a phase shift (lag) of approximately twomicroseconds on higher frequency signals, this being for the purpose ofenabling response to such higher frequency signal componentsnotwithstanding the several microsecond delay introduced in the polarityand amplitude sensing of the signal. Filter network 65 is followed by anormalizer including two attenuation networks 91 and 93 each of whichcomprises a pair of resistors connected in voltage divider relation,with the divider circuit being completed to ground through one of twoFET switches 95 and 97. Operation of these switches is controlled by thenormalization signals derived from the omni antenna signal amplitude fordynamic range limiting as will be explained. The values of resistance individer network 91 are chosen to divide the signal by a first ratio suchas five to one when the FET transistor switch 95 is on; the values ofresistance in divider network 93 are chosen to provide a second andhigher attenuation ratio such as 25 to one when the FET transistorswitch 97 also is An operational amplifier 99 couples the X channelsignal after amplitude control by the attenuation networks 91 and 93 tothe absolute value amplifier at 79.

Amplifier 99 is a high input impedance noninverting amplifier with gaincontrol provided by adjustment of the negative feedback ratio, forpurposes of matching amplifier gains as between the X and Y signalchannels.

Referring now to the normalization detector circuit 41 which suppliesrange switch control signals to the FET switches 95 and 97, the omniantenna input at 101 is coupled to he base of a transistor amplifier 103and to a zener diode 105 connected to limit transistor base signalamplitude at a level selected to be sufficiently high for control of therange switching circuitry but not so high as to permit damage to circuitcomponents due to excessive current levels therein. The output signal onthe transistor collector is coupled through a resistor 107 which isbypassed by diode 108 and capacitor 109 to a pair of resistors 111 and113 which respectively connect through tunnel diodes 115 and 117 toground, and through resistors 119 and 121 to the base electrodes oftransistors 123 and 125. The value of resistance element 113, for theapproximately to 1 difference of input voltages at which the low andhigh range switches 95 and 97 are respectively to be brought intooperation, should be approximately five times that of resistance element111; for example, resistance values of 3,300 ohms for resistor 113 and750 ohms for resistor 111 would be suitable. Resistors 119 and 121 arefor the purpose of providing bias for transistors 123 and 125,respectively, and may be of equal value typically of the order of l,000ohms.

In the absence of signal input at terminal 101, both of the tunneldiodes 115 and 117 will be off and transistors 123 and 125 also will bebiased to their off state. When signal is received at terminal 101, thecollector of transistor 103 will go negative and, depending on theamplitude of the received signal, either or both of the tunnel diodes115 and 117 will switch on. This will establish a current flow throughthe tunnel diode, the associated resistor 111 or 113, capacitor 109,diode 108 and transistor 103. The voltage across the tunnel diode 115will increase from the relatively low value which exists under quiescentconditions to a value sufficiently high to forward bias the transistor123, turning it on and switching the associated FET transistor switch 95also on. As the amplitude of the input signal further increases, tunneldiode 117 next will switch on and, in similar manner, cause transistor125 to switch on energizing the second range switch 97. Zener diode 105limits the maximum swing of the collector voltage of transistor 103,thus limiting the maximum current through tunnel diodes 115 and 117 soas to prevent possible damage thereto.

The X and Y channel signals, after normalization and amplification intheir respective absolute value amplifiers, are applied to the inputsofa dual sided operational amplifier 127 which constitutes the principalcomponent of magnitude comparator 43. When the absolute value X signal(the [X] signal in FIG. 4) is more positive than the [Y] signal, theoutput of amplifier 127 will be high and when the converse relationshipexists and the [Y] signal is the larger the amplifier output will below. The comparator output is sensed by a hold circuit comprising aflip-flop 161 which, when enabled by Time Pulse No. 2 to its clockinput, will output either a logic X Y or X Y signal depending uponwhether the output from amplifier 127 is then high or low. This logicsignal is held through the remainder of the operating cycle irrespectiveof later change in the output of amplifier 127.

The logic X Y and X Y signals thus derived are coupled via leads 129 and130 to the octant selector of FIG. 5 for use therein as later to bedescribed, and the X Y signal also is applied to the base electrode of atransistor 131 which forms part of the reversing switch drive circuit49. When the X Y signal is high this turns off transistor 131 causingthe voltage at its collector to go negative, thus switching off a secondtransistor 133 to the base electrode of which this collector signal fromtransistor 131 is coupled. The collector voltage of transistor 133 thenrises to the positive supply voltage.

The collector voltage of transistor 131 is applied through diodes 135and 136 to the gate electrodes of a pair of PET transistor switches 138and 139, respectively, and the collector voltage of transistor 133 issimilarly applied through diodes 141 and 142 to the gate electrodes ofPET transistor switches 144 and 145, respectively. The arrangement issuch that when the logic X Y signal is high, transistor 131 switches offand holds FET transistor switches 138 and 139 open so that no Y signalcircuit is completed through switch 138 and no X signal circuit iscompleted through switch 139. When transistor 131 switches offtransistor 133 follows, and its positive-going collector voltage appliedto the gate electrodes of each of the transistor switches 144 and 145causes those switches to close, thus coupling the X channel signal tooutput lead 147 and the Y channel signal to output lead 149.

Conversely, when the logic X Y signal is low, transistors 131 and 133both will be on; the collector voltage of transisotr 131 will berelatively more positive and that of transistor 133 relatively morenegative, so switches 138 and 139 now will be closed and switches 144and 145 open. Under these conditions the [Y] signal will be applied tooutput lead 147 and the [X] signal will appear on lead 149. In this waythese elements serve as a reversing switch operated in response to thecomparison of the relative magnitudes of the X and Y channel signals, toconnect whichever is the larger of those two signals to lead 147 and toconnect the other to lead 149. This invariant relationship of relativemagnitudes of the [X] and [Y] signals as applied to leads 147 and 149 isfor the purpose of facilitating operation of the X/Y ratioing circuit ofFIG. 5 next to be described.

With reference then to that figure, there is shown the octant selector31, BCD up-down counter 47, the XY ratio detector and A/D convertor 53,and positive level detector 55. As will be recalled, the sequence ofoperations resulting in the setting of the BCD up-down counters to thedesired azimuthal angle involves first a quadrant selection on the basisof relative polarities of the X and Y channel signals, next an octantselection on the basis of the relative magnitudes of the [X] and [Y]signals, and finally a degrees-within-octant determination on the basisof a ratioing of the relative magnitudes of the [X] and [Y] signals.

The quadrant and octant selection steps are accomplished by thecircuitry shown within block 31, th e octant selector, having as itsinputs the X, Y, X and Y signals from the X and Y channel polaritydetectors, and the X Y and X Y logic signals which are supplied from themagnitude comparator 43. These X and Y polarity and relative magnitudesignals are applied to a logic network designated generally by referencenumeral 151 and comprising a plurality of logic AND elements connectedas shown to provide an octant preset input to the units and tens counterelements 153 and 155 of the BCD up-down counter 47 in accordance withthe truth table of FIG. 6 as will be more fully explained hereinafter.The X Y and X Y inputs also are applied to this logic network 151 in thearrangement shown to provide an octant preset input to the counters 153and 155, again in accordance with the truth table of FIG. 6. As furtherindicated by the truth table, logic network 151 serves also to presetthe direction of count of the counters 153 and 155 upon input of serialdata on lead 157 from the X/Y ratio detector 53, i.e., the degrees databy which the angle within the preselected octant is entered into thecounters. Readout of the preset signals is controlled by a PARALLEL LOADinput on lead 159 derived as will later be explained.

Ratioing of the [X] and [Y] signal amplitudes is accomplished bydetector 53, to which the X and Y absolute value signals are inputted onleads 147 and 149 from the reversing switch 51 of FIG. 4. It will berecalled that the greater of the [X] and [Y] signals always is impressedon lead 147 and the smaller always on lead 149 as indicated. The smallersignal on lead 147 is applied directly to one of the two inputs of eachof a band of nine dual-sided operational amplifiers 171-179; lead 147applied its larger signal to the other input to each of these amplifiersthrough an associated resistance divider network the resistance valuesin each of which are stepped so as to couple to each of the amplifiers171-179 a different fraction of the input voltage ranging fromapproximately one-ninth of the input voltage up to nearly its fullvalue.

Each of the amplifiers 171-179 will output a high signal when its inputthrough the associated divider network equals or exceeds in magnitudethe signal directly applied to its other input. Thus these amplifiersserve as threshold detectors having progressively stepped thresholdvalues at which they fire, and the number of detectors which will firein response to any given pair of inputted XY signals constitutes adirect measure of the numberical value of the ratio of thos signals.Such ratio in turn constitutes a direct measure of the tangent of theazimuthal angle of the signal source within the octant. The value ofthis tangent, which will vary from zero to one across the octant, isoutputted into a ninestage shift register 191 for conversion to serialdata form for subsequent application to the up-down counters.

With a bank of nine voltage dividers and operational amplifiers asshown, the 45 of each octant would be divided into nine increments of 5each, so that the angle readout would provide a resolution of 5. Thesubsequent processing of this angle data can be simplified by avoidingthe ratios corresponding to angles of and 45, and instead using 2.5 and425 as the lower and upper extreme values and stepping in incrementstherebetween. Thus the ratio of the two resistances of the dividernetwork across which the input to the amplifier is taken is madenumerically equal to the tangent of the angle 25 for amplifier 171, theangle 7.5 for amplifier 172, and so on in 5 steps up to 42.5 foramplifier 179.

As will be appreciated, the 5 resolution which the nine-stage ratiodetector shown provides may be improved if desired simply by increasingthe number of amplifier and shift register stages, further dividing the45 of the octant to achieve as fine resolution as may be needed. It willalso be appreciated that if preferred, the individual resistance dividernetworks 181-189 could be replaced with a single divider with tapscorresponding to the desired fractions of the input voltage, though inpractice the greater flexibility offered by the individual dividerarrangement shown has been found often preferable.

The timing and sequencing of operations of the digital comparator andcounters is controlled by positive level detector 55 which has as itsinputs the X and Y signals on leads 147 and 149. These signals aretransmitted through summing resistors 193 and 195 so as to apply theirmean value to one of the two inputs of a dual-sided operationalamplifier 197 having applied to its other input a reference voltage offixed value, corresponding to the threshold signal level at which theratio detection operation is to be begun. When this threshold isexceeded the amplifier will turn on, driving its output low and applyingsuch output to one of the inputs of an inverted OR gate 201.

One of the other two inputs to this OR gate 201 is Time Pulse No. 3.This time pulse serves to trigger the ratio detection and readoutoperations in the event the positive level detector threshold valueshould for any reason not have been attained when such time period haslapsed. Any possible hangup in system operation which might otherwiseresult is thus avoided, and completion of readout is assured once anoperating cycle is begun. Time Pulse No. 3 preferably is set to occur ator near the second half-cycle peak, though it may if desired be set tooccur at any other time during the second half-cycle at which thesignals in the two channels are above the noise.

The third input of OR gate 201 is from the 0" terminal of a flip-flop203 having a CLEAR signal as a low input to its resetterminal. ThisCLEAR signal input goes high coincidentally with Time Pulse No. 2, atwhich time flip-flop 203 is caused to set its output on lead 205 to a land its output on lead 207 to a 0. The flip-flop then remains in thisstate until after application of a signal to its setinput. It will beappreciated that initially, while the CLEAR signal input is low,flip-flop 203 may output a 0 on lead 204 to gate 201 and that this mayresult in an output therethrough, but such output is to an AND gate 209the other input to which is the CLEAR signal. There accordingly is nooutput from this second gate 209 until the CLEAR signal goes high.

When the output of threshold detector amplifier 197 goes low, or whenTime Pulse No. 3 is received if that occurs first, such input to theinverted OR gate 201 will be passed to AND gate 209 and, since the otherinput to AND gate 209 is the CLEAR signal which now will be high, thegate will output on lead 211 a signal which is applied to the shiftregister 191 as a MODE CON- TROL signal and the the up-down counters 153and as a PARALLEL LOAD signal. At the same time, this signal is appliedto the set" input of flip-flop 203 which then outputs a l on its lead207 to the enable" circuit 57. This starts the clock 59, producing aclock input to shift register 191 and to an AND gate 213 through whichthe shift register output serial data is transmitted through the octantselector to the updown counters. Flip-flop 203 outputs a 0 on lead 205to the OR gate 201 to provide a continuing output therethrough which istransmitted through AND gate 209 back to the set input of the flip-flop,latching it in this state until completion of the readout operation assignalled by termination of the CLEAR pulse.

Thus when the ratio detection and readout operation is triggered eitherby threshold detector amplifier 197 or by Time Pulse No. 3, the MODECONTROL signal on lead 211 to the shift register 191 will enable theentry therein of the arc tangent angle data generated by the XY ratiodetector. At the same time this signal as applied to the BCD up-downcounters 153 and 155 will enable the parallel load therein of thequadrant and octant data derived by the octant selector 31 from its X,Y, X, Y, X Y and X Y inputs, thus presetting the counters to countsrepresenting the octant angle preselected in accordance with the truthtable of FIG. 6. This signal on lead 211 also trips flip-flop 203 andthe resulting one signal on lead 207 starts clock 59 to shift the arctangent angle data out of the shift register 191 through AND gate 213 tooctant selector 31. As flipflop 203 reverses state, the then applied toinverted OR gate 201 provides a continuing output on lead 211 for theduration of the CLEAR pulse input thus affording adequate time forcompletion of readout.

The serial data output from shift register 191 is applied to one or theother of the up and down count inputs of the BCD counters in accordancewith the direction of count derived as indicated in the truth table ofFIG. 6 from the X, Y, X, Y, X Y and X Y inputs to octant selector 31. Asthe units counter 153 counts up any carry" count is applied to the tenscounter 155, and on count down any borrow count is applied to the unitscounter 153 in conventional fashion.

As an example of the operation for presetting the BCD up-down counters,if it is assumed that the variables X, Y, and X Y all are logic onescorresponding to a source azimuthal angle of 45 to 90, logic network 151is structured to apply a logic 1 to inputs A" of the tens counter and Dof the units counter, and a logic 0 to all other inputs. The counterthus is preset to a BCD representation of 90. Gate 221 is an AND gateand since its inputs X and X Y are logic 1" it will output a l to the Dinput of units counter 153. Gate 223 is an inverted OR gate and, inresponse to its X Y input which is a logic 0, will output a logic 1" tothe A" input of tens counter 155.

As indicated by the truth table, the direction of count for an azimuthangle in the octant between 45and 90is down, the down count beginningfrom the preset value of 90. Since in this example X 0, Y 0 and X Y,these inputs all are logic l and the AND function of the four gates225-228 preceding gate 229 is not met, so its output will be a logicThis signal is inverted at its input to gate 231 and enables that gate.Clock pulses are then fed through the gate to the COUNT DOWN input ofthe units counter 153. Since the output of gate 229 is low and the ANDfunction of gate 233 is inhibited and no clock pulses are fed to theCOUNT UP input of the counter.

The other seven possible settings of the BCD updown counters can betraced through the logic network 151 as in the previous example byfollowing the truth table. The BCD up-down counters form functionally anarithmetic unit and the usual mechanisms for performing addition andsubtraction are included. The arithmetic operation to be performed iscontrolled by the COUNT UP and DOWN inputs, and borrow and carry"connections between the units counter and the tens counter are providedin a conventional fashion. The BCD counter 155 are provided in aconventional fashion. The BCD counter data may be outputted to anyappropriate recording or display device, and indicates directly theazimuthal angle of the source of the signal input to the system.

From the foregoing description of the invention it will be apparent thatthe ambiguity-free measure of azimuthal angle which the direction finderof the invention affords may by appropriate selection of the number ofstages in the ratio detector and associated signal processing circuitry,provide a desired degree of resolution while still maintaining anambiguity-free output. It will also be appreciated that the inventioncould be implemented entirely in analog form if this were preferred, andthat many different forms of both analog and digital implementationpresent useful alternatives to the particular implementation shown.

Thus while in this description of the invention only certain presentlypreferred embodiments have been illustrated and described by way ofexample, many modifications will occur to those skilled in the art andit therefore should be understood that the appended claims are intendedto cover all such modifications as fall within the true spirit and scopeof the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A direction finder for determining the azimuthal location of a sourceof wave energy incident upon the finder, comprising:

a. first wave energy sensor means having a directional sensitivitypattern disposed along a first axis and providing a first directionalsignal of magnitude proportioned to the cosine of the angle between theenergy source azimuth and said first axis;

b. second wave energy sensor means having a directional sensitivitypattern disposed along a second axis orthogonally related to said firstaxis and providing a second directional signal of magnitude proportionedto the sine of the angle between the energy source azimuth and saidfirst axis;

0. means for detecting simultaneously the polarities of each of saidfirst and second directional signals and for deriving from theirpolarities and indication of the quadrant in which said wave energysource is located;

d. means for comparing the amplitudes of said first and seconddirectional signals to determine which has the larger absolute value andfor deriving from their relative values an indication of the octantwithin said quadrant in which said wave energy source is located;

e. means for ratioing the magnitudes of said first and seconddirectional signals and for deriving thereby an indication of thetangent of the angle within said octant at which said energy source islocated; and means for combining the indications thus derived of saidfirst and second directional signal polarities and relative amplitudesand the ratio of their magnitudes, to yield a measure of the azimuthalangle of said wave energy source.

2. A direction finder as defined in claim 1 further comprising thirdwave energy sensor means having an omnidirectional sensitivity pattern,and means responsive to the nondirectional signal provided thereby tocontrol the sequence and timing of operation of said polarity detectionmeans, amplitude comparison means and magnitude ratioing means for saidfirst and second directional signals.

3. A direction finder as defined in claim 2 wherein said control meansis responsive to said nondirectional signal when of one polarity and oflevel exceeding a first threshold value to initiate said polaritydetection and amplitude comparison operations in sequence, and furtherincluding means responsive to signals of opposite polarity and of levelexceeding a second threshold value to initiate said magnitude ratioingoperation, whereby signal polarity detection and amplitude comparisonare accomplished during one half-cycle of the nondirectional signal andmagnitude ratioing is accomplished during the following half-cycle.

4. A direction finder as defined in claim 1 further comprising thirdwave energy sensor means having an omnidirectional sensitivity pattern,and means responsive to the nondirectional signal provided thereby tonormalize said first and second directional signals equally andsimultaneously.

5. A direction finder as defined in claim 4 wherein said signalnormalization means comprises a like pair of switchable attenuators eachcontrolling the signal level of one of said first and second directionalsignals, and electronic switching means responsive to amplitude of saidnondirectional signal to control both said attenuators so that the firstand second directional signals thus normalized each represent anidentical fraction of the corresponding signal before normalization, andthe larger of said first and second directional signals thus normalizedremains within predetermined dynamic range limits.

6. A direction finder as defined in claim 1 wherein said means forcombining said derived indications of signal polarities, relativeamplitudes and ratio of magnitudes, comprise digital counter means,means responsive to said signal polarity and relative amplitudeindications to preset into said counter means digital valuesrepresenting the quadrant and octant in which said wave energy source islocated and to determine the direction of count from such preset valuesto the azimuthal angle of said source, and means responsive to saidmagnitude ratio indication to cause said counter means to count in thedirection thus determined and through a number of counts representingthe azimuthal angle of said wave energy source within the octant thuspreset.

7. A direction finder as defined in claim 1 further including reversingswitch means through which said first and second directional signals aretransmitted to said means for ratioing their magnitudes, and drive meansfor said switching means operative under control of said signalamplitude comparison means to apply said directional signals to saidratioing means in a manner such that the larger amplitude signal isratioed into the smaller.

8. A direction finder for determining the azimuthal angle to a source ofwave energy incident upon the finder, comprising:

a. first and second wave energy sensor means each having a directionalsensitivity pattern which is orthogonally related to that of the otherand each providing a directional signal which together with that of theother is determinative of the location of the energy source azimuth asreferenced to the respective directions of sensitivity of said sensors;

b. third wave energy sensor means having an omnidirectional sensitivitypattern and providing a nondirectional signal;

0. means responsive to said nondirectional signal to produce a series oftiming signals sequenced in time;

d. means enabled by a first of said timing signals for detectingsimultaneously the polarities of each of said directional signals andfor deriving from their relative polarities an indication of thequadrant in which said wave energy source is located;

e. means enabled by a second of said timing signals for comparing theamplitudes of said directional signals to determine which has the largerabsolute value and for selecting the octant within said quadrant inwhich said wave energy source is located;

f. means enabled by a third of said timing signals for ratioing themagnitudes of said directional signals and for deriving thereby anindication of the angle between azimuth aximuth on which said energysource is located and one of the two azimuth on which said energy sourcetogether define said selected octant; and

3. means for combining the indications thus derived of the relativepolarities and amplitudes of said directional signals to identify saidone azimuth and the direction therefrom of said energy source azimuth,and for adding thereto the angle indicated by ratioing means to yield ameasure of the azimuthal angle to said wave energy source.

9. A direction finder as defined in claim 8 further comprising meansresponsive to said nondirectional signal to normalize both saiddirectional signals simultaneously and equally.

10. A direction finder as defined in claim 8 further comprising meansfor sequencing said timing signals so that said polarity detection andamplitude comparison means are enabled thereby during one halfcycle ofsaid nondirectional signal and said magnitude ratioing means are enabledthereby during the following halfcycle.

